CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
8.3 DIFFICULTIES WITH CHEMICAL FORMULAE AND EQUATIONS
8.3.1 Difficulties with formulae for acids and bases
8.3.1.1 Difficulty R6: All formulae with hydrogen indicate acids
A considerable number of students in senior secondary and even tertiary education treat formulae very superficially (see Table 8.2 below). The same difficulty has been reported in a wide variety of chemical contexts, all concerning bases with hydrogen in the formula. The formulae investigated include that for phosphine, PH3, (a typical Lewis base and a weak Brønsted base), ammonia, NH3 (a typical Brønsted base), methylamine, CH3NH2 (a weak Brønsted base) and even sodium hydroxide, NaOH, (a prototypic Arrhenius base). All were identified to be acids by the students as in the following typical quotation concerning PH3: "it contains hydrogens and, therefore, can provide [H+]... in aqueous solution" (Zoller, 1996).
Table 8.2 Summary of research concerning formulae incorrectly classified as acids
Formula investigated
Percentage students classifying compound as an acid
Educational level of students
Country of cohort
Author(s)
PH3 not applicable Tertiary Israel Zoller (1996)
NH3 42 Senior secondary Spain Furió-Más et al.
(2007)
NH3 55 Senior secondary Tunisia Ouertatani et al.
(2007)
CH3NH2 55 Senior secondary Spain Furió-Más et al.
(2007)
NaOH 24 Senior secondary Spain Furió-Más et al.
(2007)
NaOH 10-15 Senior secondary Tunisia Ouertatani et al.
(2007)
The evidence for this difficulty in Table 8.2 shows its widespread occurrence among students from different language groups in many parts of the world, and its high incidence cannot be ignored, for example up to 55% of senior secondary students thought ammonia was an acid.
From this evidence and without further analysis, the difficulty can be described as: All formulae with hydrogen indicate acids. The difficulty description is applicable through all the chemical and educational contexts in the table, so it can be classified as Level 4 or Established. The student difficulty maps to propositional statements below, of which the first two were introduced in Chapter 6 and are modified to include examples investigated above:
• Arrhenius bases include NaOH, Al(OH)3 and Zn(OH)2 (3.2.2.1.1)
• Brønsted bases: examples include the molecules H2O, NH3, PH3, amines and ions OH-, HCOO–, CO3
2–, HCO3
–, HSO4 –, SO4
2–, HS–, CN– and S2–(3.3.2.1)
• Amines are organic bases with a functional group –NH2 such as CH3NH2 (3.3.2.1.1.1)
8.3.1.2 Sub-Difficulty R6.1 Bases have formulae with no hydrogen
A corollary to Difficulty R6 concerns the notion that bases have no hydrogen. Nakhleh and Krajcik (1994) report a single student quote about bases having formulae with no hydrogen.
With no further controlled research into the nature of the difficulty, it is classified as a Level 1 or Suspected difficulty. This difficulty could be due to confused thinking about Brønsted bases as proton acceptors and, therefore, maps to the same set of examples given as propositional statement 3.3.2.1 for R6 above. For this reason, it is not treated as a separate difficulty.
8.3.1.3 Difficulty R7: All formulae with an OH group indicate bases.
In the context of bases, students display the same superficial student reasoning as they did with respect to acids. The reasoning is typified by the following student quotation concerning CH3OH: “an ionic substance and so the OH– in the formula is a hydroxide ion” (Furió-Más et al., 2007). As with the previous difficulty, evidence in support of this difficulty comes from a wide variety of sources in educational contexts worldwide, mostly concerning senior secondary students (see Table 8.3 below).
Table 8.3 Summary of research concerning formulae incorrectly classified as bases
Formula investigated
Percentage students classifying compound
as a base
Educational level of students
Country of cohort
Author(s)
O=P(OH)3 not applicable Tertiary Israel Zoller (1996)
CH3CH2OH 26 – 33 Senior secondary Tunisia Ouertatani et al. (2007) CH3CH2OH 27 & 29 respectively Junior & Senior
secondary
Taiwan Chiu (2007)
CH3OH 61 Senior secondary Spain Furió-Más et al. (2007)
CH3COOH 2 Senior secondary Greece Kousathana et al. (2005)
CH3COOH 27 Senior secondary Spain Furió-Más et al. (2007)
Compounds investigated were phosphoric acid (usually given with the formula H3PO4, but here represented as O=P(OH)3), ethanol, CH3CH2OH, and methanol, CH3OH, (alcohols, not bases) and ethanoic (commonly called acetic) acid which has a typical carboxylic acid group COOH.
As with the previous difficulty, the evidence needs no further analysis to derive the difficulty description given above.
While the cause for this difficulty may lie with underlying knowledge of bonding (Furió-Más et al., 2007), the propositional knowledge statements below, arising from the chemical examples above, are relevant in the acid-base context .
• Arrhenius acids: examples include HCl, H3PO4 (sometimes given as O=P(OH)3) and carboxylic acids (2.2.2.1.1)
• Carboxylic acids are organic compounds with a functional group –COOH (2.2.2.1.2) for example: CH3COOH, and HCOOH. (2.2.2.1.2.1)
• Arrhenius bases: examples do not include alcohols (3.2.2.2.2)
• Alcohols have the functional group –OH (5.2.2) for example: CH3OH and CH3CH2OH (5.2.2.1)
Based on the many different chemistry contexts investigated, as well as the consistency in student responses, I could classify the difficulty as Established or at Level 4. With fairly high incidences of the difficulty, as with R6, it cannot be ignored when teaching. Moreover, all the evidence together, suggests further questions. However, frustratingly, no further information is available to explain for instance why junior secondary students in Taiwan outperform their senior secondary counterparts. Neither do we know whether the much lower incidence of the difficulty in the Greek cohort (2%) was a result of a particular teaching strategy or whether the Spanish students had yet not been taught about carboxylic acids. The evidence does, however, suggest some answers below to questions arising from other prior research.
Ye and Wells (1998) had also investigated student conceptions of chemical formulae through multiple-choice questions. For the stem: “The formulas for the most common organic bases end in…” they found that many students chose the distractor COOH from the other options: Cl–, NH2, and H2O. The authors speculate that students linked the word ‘organic’ to the only choice that involved a carbon atom but did not show any data to substantiate this interpretation. In the light of the research shown above, it is more likely to have been an association of the OH group with bases, rather than the carbon atom which enticed the students. The rather glib interpretation from Ye and Wells illustrates the lack of insight gained through multiple-choice instruments if distractors are not based on prior, open-ended research.
8.3.1.4 Discussion of difficulties with formulae for acids and bases
The descriptions for difficulties R6 and R7 both appear to be linked by simplistic reasoning leading some researchers (e.g. Zoller, 1996; Lin & Chiu, 2007) to describe them as one difficulty. Should they in fact be considered as one difficulty? As described in the Methods chapter (Section 4.5) difficulties are considered separate if they have different causes, different educational implications or if they need to be addressed through different teaching strategies.
An analysis of the two difficulty descriptions in the light of this reasoning follows next.
From the theoretical framework (see Section 3.3) for the three historical models considered here, all acids contain hydrogen, and all Arrhenius acids are also Brønsted acids, but not vice versa. By contrast, Brønsted bases and Arrhenius (or operational model) bases are mutually exclusive classifications. This means that an Arrhenius base cannot be a Brønsted base, and neither can a Brønsted base be an Arrhenius base. It thus follows that a student with Difficulty R6 who uses the heuristic: All formulae with hydrogen indicate acids will not necessarily have a misplaced idea of what constitutes an acid in any of the models, although they treat formulae superficially. However, the conception R7: All formulae with OH indicate bases suggests not only a simplistic way of looking at formulae, but a circumscribed conception of a base, allowing only the Arrhenius model. Thus, it is proposed that R6 and R7 should be seen as two separate difficulties.
Students could fruitfully add examples to the categories illustrated in Figure 8.1 which follows.
This diagram shows that all of the Arrhenius acids such as HCl, H2SO4 and H3PO4 are also Brønsted acids, whereas bases which are common to both the Arrhenius and Brønsted models are seldom included in high school curricula (see Section 3.3.3.4). Furthermore there are some examples of that fall into acids and bases, these are amphoteric species.
Figure 8.1 Classification of examples of acids and bases